Methods of making low-shrink polypropylene fibers

ABSTRACT

Improved polypropylene fibers exhibiting greatly reduced heat- and moisture-shrink problems and including certain compounds that quickly and effectively provide rigidity to the target polypropylene fiber after heat-setting are disclosed herein. In such a manner, the “rigidifying” compounds provide nucleation sites for polypropylene crystal growth. After drawing the nucleated composition into fiber form, the fiber is then exposed to sufficient heat to grow the crystalline network, thus holding the fiber in a desired position. The preferred “rigidifying” compounds include dibenzylidene sorbitol based compounds, as well as less preferred compounds, such as sodium benzoate, certain sodium and lithium phosphate salts (such as sodium 2,2′-methylene-bis-(4,6-di-tert-butylphenyl)phosphate, otherwise known as NA-11).

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] This application is a continuation of co-pending application Ser.No. 09/860,130, filed on May 17, 2001. This parent application is hereinentirely incorporated by reference.

FIELD OF THE INVENTION

[0002] This invention relates to improvements in preventing heat- andmoisture-shrink problems in specific polypropylene fibers. Such fibersrequire the presence of certain compounds that quickly and effectivelyprovide rigidity to the target polypropylene fiber after heat-setting.Generally, these compounds include any structure that nucleates polymercrystals within the target polypropylene after exposure to sufficientheat to melt the initial pelletized polymer and upon allowing such amelt to cool. The compounds must nucleate polymer crystals at a highertemperature than the target polypropylene without the nucleating agentduring cooling. In such a manner, the “rigidifying” nucleator compoundsprovide nucleation sites for polypropylene crystal growth. After drawingthe nucleated composition into fiber form, the fiber is then exposed tosufficient heat to grow the crystalline network, thus holding the fiberin a desired position. The preferred “rigidifying” compounds includedibenzylidene sorbitol based compounds, as well as less preferredcompounds, such as sodium benzoate, certain sodium and lithium phosphatesalts (such as sodium2,2′-methylene-bis-(4,6-di-tert-butylphenyl)phosphate, otherwise knownas NA-11). Specific methods of manufacture of such fibers, as well asfabric articles made therefrom, are also encompassed within thisinvention.

DISCUSSION OF THE PRIOR ART

[0003] There has been a continued desire to utilize polypropylene fibersin various different products, ranging from apparel to carpet backings(as well as carpet pile fabrics) to reinforcement fabrics, and so on.Polypropylene fibers exhibit excellent strength characteristics, highlydesirable hand and feel, and do not easily degrade or erode when exposedto certain “destructive” chemicals. However, even with such impressiveand beneficial properties and an abundance of polypropylene, which isrelatively inexpensive to manufacture and readily available as apetroleum refinery byproduct, such fibers are not widely utilized inproducts that are exposed to relatively high temperatures during use,cleaning, and the like. This is due primarily to the high and generallynon-uniform heat- and moisture-shrink characteristics exhibited bytypical polypropylene fibers. Such fibers are not heat stable and whenexposed to standard temperatures (such as 150° C. and 130° C.temperatures), the shrinkage range from about 5% (in boiling water) toabout 7-8% (for hot air exposure) to 12-13% (for higher temperature hotair). These extremely high and varied shrink rates thus render theutilization and processability of highly desirable polypropylene fibersvery low, particularly for end-uses that require heat stability (such asapparel, carpet pile, carpet backings, molded pieces, and the like). Todate, there has been no simple solution to such a problem. Some ideashave included narrowing and controlling the molecular weightdistribution of the polypropylene components themselves in each fiber ormechanically working the target fibers prior to and during heat-setting.Unfortunately, molecular weight control is extremely difficult toaccomplish initially, and has only provided the above-listed shrinkrates (which are still too high for widespread utilization within thefabric industry). Furthermore, the utilization of very high heat-settingtemperatures during mechanical treatment has, in most instances,resulted in the loss of good hand and feel to the subject fibers.Another solution to this problem is preshrinking the fibers, whichinvolves winding the fiber on a crushable paper package, allowing thefiber to sit in the oven and shrink for long times, (crushing the paperpackage), and then rewinding on a package acceptable for furtherprocessing. This process, while yielding an acceptable yarn, isexpensive, making the resulting fiber uncompetitive as compared topolyester and nylon fibers. As a result, there has not been any teachingor disclosure within the pertinent prior art providing any heat- and/ormoisture-shrink improvements in polypropylene fiber technology.

DESCRIPTION OF THE INVENTION

[0004] It is thus an object of the invention to provide improved shrinkrates for standard polypropylene fibers. A further object of theinvention is to provide a class of additives that, in a range ofconcentrations, will give low shrinkage. A further object of theinvention is to provide a specific method for the production ofnucleator-containing polypropylene fibers permitting the ultimateproduction of such low-shrink fabrics therewith. Accordingly, thisinvention encompasses a polypropylene fiber possessing at most 5,000denier per filament and exhibiting a heat-shrinkage in at least 150° C.hot air of at most 11%, wherein said fiber further comprises at leastone nucleating agent. Also, this invention encompasses a polypropylenefiber possessing at most 5,000 denier per filament and exhibiting aheat-shrinkage in at least 150° C. hot air of at most 11%, wherein saidfiber further comprises at least one nucleating agent, and wherein saidfiber further exhibits a long period of at least 20 nm as measured bysmall-angle x-ray scattering. Furthermore, this invention encompasses apolypropylene fiber possessing at most 5,000 denier per filament andcomprising at least one nucleating agent, and wherein said fiber furtherexhibits a long period of at least 20 nm as measured by small-anglex-ray diffraction spectroscopy. Additionally, this invention encompassesa polypropylene fiber possessing at most 5,000 denier per filament andexhibiting a peak crystallization temperature of at least 115° C. asmeasured by differential scanning calorimetry in accordance with amodified ASTM Test Method D3417-99 at a cooling rate of 20° C./min, andwherein said fiber further exhibits a long period of at least 20 nm asmeasured by small-angle x-ray scattering. Certain yarns and fabricarticles comprising such inventive fibers are also encompassed withinthis invention.

[0005] Furthermore, this invention also concerns a method of producingsuch fibers comprising the sequential steps of a) providing apolypropylene composition in pellet or liquid form comprising at least100 ppm by weight of a nucleator compound; b) melting and mixing saidpolypropylene composition of step “a” to form a substantiallyhomogeneous molten plastic formulation; c) extruding said plasticformulation to form a fiber structure; d) mechanically drawing saidextruded fiber (optionally while exposing said fiber to a temperature ofat most 105° C.); and e) exposing said drawn fiber of step “d” to asubsequent heat-setting temperature of at least 110° C. Preferably, step“b” will be performed at a temperature sufficient to effectuate themelting of all polymer constituent (e.g., polypropylene), and possiblythe remaining compounds, including the nucleating agent, as well(melting of the nucleating agent is not a requirement since somenucleating agents do not melt upon exposure to such high temperatures).Thus, temperatures within the range of from about 175 to about 300° C.,as an example (preferably from about 200 to about 275°, and mostpreferably from about 220 to about 250° C., are proper for this purpose.The extrusion step (“c”) should be performed while exposing thepolypropylene formulation to a temperature of from about 185 to about300° C., preferably from about 210 to about 275° C., and most preferablyfrom about 230 to about 250° C., basically sufficient to perform theextrusion of a liquefied polymer without permitting breaking of any ofthe fibers themselves during such an extrusion procedure. The drawingstep may be performed at a temperature which is cooler than normal for astandard polypropylene (or other polymer) fiber drawing process. Thus,if a cold-drawing step is followed, such a temperature should be belowabout 105° C., more preferably below about 100° C., and most preferablybelow about 90° C. Of course, higher temperatures may be used if no suchcold drawing step is followed. The final heat-setting temperature isnecessary to “lock” the polypropylene crystalline structure in placeafter extruding and drawing. Such a heat-setting step generally lastsfor a portion of a second, up to potentially a couple of minutes (i.e.,from about {fraction (1/10)}^(th) of a second, preferably about ½ of asecond, up to about 3 minutes, preferably greater than ½ of a second).The heat-setting temperature must be greater than the drawingtemperature and must be at least 110° C., more preferably at least about115°, and most preferably at least about 125° C. The term “mechanicallydrawing” is intended to encompass any number of procedures whichbasically involve placing an extensional force on fibers in order toelongate the polymer therein. Such a procedure may be accomplished withany number of apparatus, including, without limitation, godet rolls, niprolls, steam cans, hot or cold gaseous jets (air or steam), and otherlike mechanical means.

[0006] In another embodiment of the method of making such inventivefibers, step “c” noted above may be further separated into two distinctsteps. A first during which the polymer is extruded as a sheet or tube,and a second during which the sheet or tube is slit into narrow fibersof less than 5000 deniers per filament (dpf).

[0007] All shrinkage values discussed as they pertain to the inventivefibers and methods of making thereof correspond to exposure times foreach test (hot air and boiling water) of about 5 minutes. Theheat-shrinkage at about 150° C. in hot air is, as noted above, at most11% for the inventive fiber; preferably, this heat-shrinkage is at most9%; more preferably at most 8%; and most preferably at most 7%. Also,the amount of nucleating agent present within the inventive fiber is atleast 10 ppm; preferably this amount is at least 100 ppm; and mostpreferably is at least 1250 ppm. Any amount of such a nucleating agentshould suffice to provide the desired shrinkage rates after heat-settingof the fiber itself, however, excessive amounts (e.g., above about10,000 ppm and even as low as about 6,000 ppm) should be avoided,primarily due to costs, but also due to potential processing problemswith greater amounts of additives present within the target fibers.

[0008] The term “polypropylene” is intended to encompass any polymericcomposition comprising propylene monomers, either alone or in mixture orcopolymer with other randomly selected and oriented polyolefins, dienes,or other monomers (such as ethylene, butylene, and the like). Such aterm also encompasses any different configuration and arrangement of theconstituent monomers (such as syndiotactic, isotactic, and the like).Thus, the term as applied to fibers is intended to encompass actual longstrands, tapes, threads, and the like, of drawn polymer. Thepolypropylene may be of any standard melt flow (by testing); however,standard fiber grade polypropylene resins possess ranges of Melt FlowIndices between about 2 and 50. Contrary to standard plaques,containers, sheets, and the like (such as taught within U.S. Pat. No.4,016,118 to Hamada et al., for example), fibers clearly differ instructure since they must exhibit a length that far exceeds itscross-sectional area (such, for example, its diameter for round fibers).Fibers are extruded and drawn; articles are blow-molded or injectionmolded, to name two alternative production methods. Also, thecrystalline morphology of polypropylene within fibers is different thanthat of standard articles, plaques, sheets, and the like. For instance,the dpf of such polypropylene fibers is at most about 5000; whereas thedpf of these other articles is much greater. Polypropylene articlesgenerally exhibit spherulitic crystals while fibers exhibit elongated,extended crystal structures. Thus, there is a great difference instructure between fibers and polypropylene articles such that anypredictions made for spherulitic particles (crystals) of nucleatedpolypropylene do not provide any basis for determining the effectivenessof such nucleators as additives within polypropylene fibers.

[0009] The terms “nucleators”, “nucleator compound(s)”, “nucleatingagent”, and “nucleating agents” are intended to generally encompass,singularly or in combination, any additive to polypropylene thatproduces nucleation sites for polypropylene crystals from transitionfrom its molten state to a solid, cooled structure. Hence, since thepolypropylene composition (including nucleator compounds) must be moltento eventually extrude the fiber itself, the nucleator compound willprovide such nucleation sites upon cooling of the polypropylene from itsmolten state. The only way in which such compounds provide the necessarynucleation sites is if such sites form prior to polypropylenerecrystallization itself. Thus, any compound that exhibits such abeneficial effect and property is included within this definition. Suchnucleator compounds more specifically include dibenzylidene sorbitoltypes, including, without limitation, dibenzylidene sorbitol (DBS),monomethyldibenzylidene sorbitol, such as1,3:2,4-bis(p-methylbenzylidene) sorbitol (p-MDBS), dimethyldibenzylidene sorbitol, such as 1,3:2,4-bis(3,4-dimethylbenzylidene)sorbitol (3,4-DMDBS); other compounds of this type include, again,without limitation, sodium benzoate, NA-11, and the like. Theconcentration of such nucleating agents (in total) within the targetpolypropylene fiber is at least 100 ppm, preferably at least 1250 ppm.Thus, from about 100 to about 5000 ppm, preferably from about 500 ppm toabout 4000 ppm, more preferably from about 1000 ppm to about 3500 ppm,still more preferably from about 1500 ppm to about 3000 ppm, even morepreferably from about 2000 ppm to about 3000 ppm, and most preferablyfrom about 2500 to about 3000 ppm. Furthermore, fibers may be producedby the extrusion and drawing of a single strand of polypropylene asdescribed above, or also by extrusion of a sheet, then cutting the sheetinto fibers, then following the steps as described above to draw,heat-set, and collect the resultant fibers. In addition, other methodsto make fibers, such as fibrillation, and the like, are envisioned forthe same purpose.

[0010] Also, without being limited by any specific scientific theory, itappears that the shrink-reducing nucleators which perform the best arethose which exhibit relatively high solubility within the propyleneitself. Thus, compounds which are readily soluble, such as1,3:2,4-bis(p-methylbenzylidene) sorbitol provides the lowest shrinkagerate for the desired polypropylene fibers. The DBS derivative compoundsare considered the best shrink-reducing nucleators within this inventiondue to the low crystalline sizes produced by such compounds. Othernucleators, such as NA-11, also provide good low-shrink characteristicsto the target polypropylene fiber; however, apparently due to poordispersion of NA-11 in polypropylene and the large and varied crystalsizes of NA-11 within the fiber itself, the shrink rates are noticeablyhigher than for the highly soluble, low crystal-size polypropyleneproduced by well-dispersed MDBS.

[0011] One manner of testing for the presence of a nucleating agentwithin the target fibers is preferably through differential scanningcalorimetry to determine the peak crystallization temperature exhibitedby the resultant polypropylene. The fiber is melted and placed betweentwo plates under high temperature and pressure to form a sheet of sampleplastic. A sample of this plastic is then melted and subjected to adifferential scanning calorimetry analytical procedure in accordancewith modified ASTM Test Method D3417-99 at a cooling rate of 20°C./minute. A sufficiently high peak crystallization temperature (aboveabout 115° C., more preferably above about 116° C., and most preferablyabove about 116.5° C.), well above that exhibited by the unnucleatedpolypropylene itself, shall indicate the presence of a nucleating agentsince attaining such a high peak crystallization without a nucleatingagent is not generally possible.

[0012] It has been determined that the nucleator compounds that exhibitgood solubility in the target molten polypropylene resins (and thus areliquid in nature during that stage in the fiber-production process)provide more effective low-shrink characteristics. Thus, low substitutedDBS compounds (including DBS, p-MDBS) appear to provide fewermanufacturing issues as well as lower shrink properties within thefinished polypropylene fibers themselves. Although p-MDBS is preferred,however, any of the above-mentioned nucleators may be utilized withinthis invention as long as the long period (SAXS) measurements are met orthe low shrink requirements are achieved through utilization of suchcompounds. Mixtures of such nucleators may also be used duringprocessing in order to provide such low-shrink properties as well aspossible organoleptic improvements, facilitation of processing, or cost.

[0013] In addition to those compounds noted above, sodium benzoate andNA-11 are well known as nucleating agents for standard polypropylenecompositions (such as the aforementioned plaques, containers, films,sheets, and the like) and exhibit excellent recrystallizationtemperatures and very quick injection molding cycle times for thosepurposes. The dibenzylidene sorbitol types exhibit the same types ofproperties as well as excellent clarity within such standardpolypropylene forms (plaques, sheets, etc.). For the purposes of thisinvention, it has been found that the dibenzylidene sorbitol types arepreferred as nucleator compounds within the target polypropylene fibers.Of interest, as well, is the ability to provide a purely liquidformulation of the dibenzylidene sorbitol compounds for introductionwithin the target polypropylene compositions. Such liquid DBSformulations comprise certain nonionic surfactants that can be selectedboth for their liquefying and stability-providing benefits to the DBScompounds themselves, but also potentially for their lubricatingproperties for the eventual fiber. In such a manner, the amount oflubricant generally required for and added to the target fiber may bereduced or eliminated, thus reducing costs associated with suchadditives. Thus, the surfactants required for such a liquid nucleatorcomposition of 3,4-DMDBS (or other types of nucleating agents), includethose which are nonionic and which are ethoxylated to the extent thattheir hydrophilic-lipophilic balance (HLB) is greater than about 8.5.HLB is a measure of the solubility of a surfactant both in oil and inwater and is approximated as one-fifth (⅕) the weight percent of ethoxygroups present on the particular surfactant backbone. More specifically,such surfactants exhibit a HLB value of more preferably greater thanabout 12, and most preferably greater than about 13, and must possess atleast some degree of ethoxylation, more preferably greater than about 4molar equivalents of ethylene oxide (EO) per molecule, and mostpreferably greater than about 9.5 molar equivalents of EO per molecule.

[0014] Of these preferred surfactants, the most preferred forutilization within the potential fluid nucleating agent dispersion forpurposes of this invention include, in tabulated form: SURFACTANT TABLEPreferred Diluent Surfactants (with Tradenames) Ex. Surfactant Availableas and From HLB # 1 sorbitan monooleate (20 EO) Tween 80 ®; ImperialChemical (ICI) 15.0 2 sorbitan monostearate (20 EO) Tween 60 ®; ICI 14.93 sorbitan monopalmitate (20 EO) Tween 40 ®; ICI 15.6 4 sorbitanmonolaurate (20 EO) Tween 20 ®; ICI 16.7 5 dinonylphenol ether (7 EO)Igepal ® DM 430; Rhône-Poulenc (RP) 9.5 6 nonylphenol ether (6 EO)Igepal ® CO 530; RP 10.8 7 nonylphenol ether (12 EO) Igepal ® CO 720; RP14.2 8 dinonylphenol ether (9 EO) Igepal ® DM 530; RP 10.6 9 nonylphenolether (9 EO) Igepal ® CO 630; RP 13.0 10 nonylphenol ether (4 EO)Igepal ® CO 430; RP 8.8 11 dodecylphenol ether (5.5 EO) Igepal ® RC 520;RP 430 9.6 12 dodecylphenol ether (9.5 EO) Igepal ® RC 620; RP 12.3 13dodecylphenol ether (11 EO) Igepal ® RC 630; RP 13.0 14 nonylphenolether (9.5 EO) Syn Fac ® 905; Milliken & Company ˜13 15 octylphenolether (10 EO) Triton ® X-100; Rohm & Haas 13.5

[0015] This list is not exhaustive as these are merely the preferredsurfactants for use within the potential fluid nucleating agentdispersion for utilization within this invention. In such a fluiddispersion, then, the nucleating agent, such as preferably 3,4-DMDBS,comprises at most 40% by weight, preferably about 30% by weight, of theentire inventive fluid dispersion. Any higher amount will deleteriouslyaffect the viscosity of the dispersion. Preferably the amount ofsurfactant is from about 70% to about 99.9%, more preferably from about70% to about 85%; and most preferably, from about 70% to about 75% ofthe entire inventive fluid dispersion. A certain amount of water mayalso be present in order to effectively lower the viscosity of theoverall liquid dispersion. Optional additives may include plasticizers,antistatic agents, stabilizers, ultraviolet absorbers, and other similarstandard polyolefin thermoplastic additives. Other additives may also bepresent within this composition, most notably antioxidants, antistaticcompounds, perfumes, chlorine scavengers, and the like. As noted above,this type of fluid dispersion is disclosed in greater detail within U.S.Pat. Nos. 6,102,999 and 6,127,440, both herein entirely incorporated byreference. Most preferred is a composition of 30% by weight of 3,4-DMDBSand 70% by weight of Tween® 80. This mixture is listed in the PreferredEmbodiments section below as “Liquid 3,4-DMDBS”.

[0016] The closest prior art references teach the addition of nucleatorcompounds to general polypropylene compositions (such as in U.S. Pat.No. 4,016,118, referenced above). However, some teachings include theutilization of certain DBS compounds within limited portions of fibersin a multicomponent polypropylene textile structure. For example, U.S.Pat. Nos. 5,798,167 to Connor et al. and 5,811,045 to Pike, both teachthe addition of DBS compounds to polypropylene in fiber form; however,there are vital differences between those disclosures and the presentinvention. For example, both patents require the aforementionedmulticomponent structures of fibers. Thus, even with DBS compounds insome polypropylene fiber components within each fiber type, the shrinkrate for each is dominated by the other polypropylene fiber componentswhich do not have the benefit of the nucleating agent. Also, there areno lamellae that give a long period (as measured by small-angle X-rayscattering) thicker than 20 nm formed within the polypropylene fibersdue to the lack of a post-heatsetting step being performed. Again, thesethick lamellae provide the desired inventive higher heat-shrink fiber.Also of importance is the fact that, for instance, Connor et al. requirea nonwoven polypropylene fabric laminate containing a DBS additivesituated around a polypropylene internal fabric layer which contained nonucleating agent additive. The internal layer, being polypropylenewithout the aid of a nucleating agent additive, dictates the shrink ratefor this structure. Furthermore, the patentees do not expose their yarnsand fibers to heat-setting procedures in order to permanently configurethe crystalline fiber structures of the yarns themselves as low-shrinkis not their objective.

[0017] In addition, Spruiell, et al, Journal of Applied Polymer Science,Vol. 62, pp. 1965-75 (1996), reveal using a nucleating agent, MDBS, at0.1%, to increase the nucleation rate during spinning. However, aftercrystallizing and drawing the fiber, Spruiell et al. do not expose thenucleated fiber to any heat, which is necessary to impart the very bestshrinkage properties, therefore the shrinkage of their fibers wassimilar to conventional polypropylene fibers without a nucleating agentadditive. In the examples below, yarn made with similar levels ofnucleating agent additives included and no further heat exposure showedworse shrinkage (at all measured temperatures after the standard 5minute exposure time) than commercial fibers, and fibers which containedno additive and were exposed to the same conditions. Thus, in additionto the presence of the nucleating agent additive, exposure to heat aftermechanical drawing is a crucial step in the invention.

[0018] Of particular interest and which has been determined to be ofprimary importance in the production of such inventive low-shrinkpolypropylene fibers, is the discovery that, at the very least, thepresence of nucleating agent within heat-set polypropylene fibers (asdiscussed herein), provides high long period measurements for thecrystalline lamellae of the polypropylene itself. This discovery is bestexplained by the following:

[0019] Polymers, when crystallized from a melt under dynamic temperatureand stress conditions, first supercool and then crystallize with thecrystallization rate dependent on the number of nucleation sites, andthe growth rate of the polymer, which are both in turn related to thethermal and mechanical working that the polymer is subjected to as itcools. These processes are particularly complex in a normal fiberdrawing line. The results of this complex crystallization, however, canbe measured using small angle x-ray scattering (SAXS), with the measuredSAXS long period representative of an average crystallizationtemperature. A higher SAXS long period corresponds to thicker lamellae(which are the plate-like polymer crystals characteristic ofsemi-crystalline polymers like PP). The higher the crystallizationtemperature of the average crystal, the thicker the measured SAXS longperiod will be. Further, higher SAXS long periods are characteristic ofmore thermally stable polymeric crystals. Crystals with shorter SAXSlong periods will “melt”, or relax and recrystallize into new, thickercrystals, at a lower temperature than those with higher SAXS longperiods. Crystals with higher SAXS long periods remain stable to highertemperatures, requiring more heat to destabilize the crystallinestructure.

[0020] In highly oriented polymeric samples such as fibers, those withhigher SAXS long periods will remain stable to higher temperatures. Thusthe shrinkage, which is a normal effect of the relaxation of the highlyoriented polymeric samples, remains low to higher temperatures than inthose highly oriented polymeric samples with lower SAXS long periods. Inthis invention, as is evident from these measurements, the nucleatingadditive is used in conjunction with a thermal treatment to createfibers with extremely high SAXS long periods of at least 20 nm, orpreferably at least 22 nm, which in turn are very stable and exhibit lowshrinkage up to very high temperatures.

[0021] Furthermore, such fibers may also be colored to provide otheraesthetic features for the end user. Thus, the fibers may also comprisecoloring agents, such as, for example, pigments, with fixing agents forlightfastness purposes. For this reason, it is desirable to utilizenucleating agents that do not impart visible color or colors to thetarget fibers. Other additives may also be present, including antistaticagents, brightening compounds, clarifyng agents, antioxidants,antimicrobials (preferably silver-based ion-exchange compounds, such asALPHASAN® antimicrobials available from Milliken & Company), UVstabilizers, fillers, and the like. Furthermore, any fabrics made fromsuch inventive fibers may be, without limitation, woven, knit,non-woven, in-laid scrim, any combination thereof, and the like.Additionally, such fabrics may include fibers other than the inventivepolypropylene fibers, including, without limitation, natural fibers,such as cotton, wool, abaca, hemp, ramie, and the like; syntheticfibers, such as polyesters, polyamides, polyaramids, other polyolefins(including non-low-shrink polypropylene), polylactic acids, and thelike; inorganic fibers such as glass, boron-containing fibers, and thelike; and any blends thereof.

BRIEF DESCRIPTION OF THE DRAWINGS

[0022] The accompanying drawings, which are incorporated in andconstitute a part of this specification, illustrate a potentiallypreferred embodiment of producing the inventive low-shrink polypropylenefibers and together with the description serve to explain the principlesof the invention wherein:

[0023]FIG. 1 is a schematic of the potentially preferred method ofproducing low-shrink polypropylene.

[0024]FIG. 2 is described in greater detail below with regard to smallangle X-ray scattering and is a graphical representation of theintegrated intensity data I(q) as a function of 2θ in order to determinethe long period spacing of the target fibers.

[0025]FIG. 3 is also described in greater detail below with regard tosmall angle X-ray scattering and is a graphical representation of theK(z) function to aid in the ultimate determination of long periodspacing.

DETAILED DESCRIPTION OF THE DRAWING AND OF THE PREFERRED EMBODIMENT

[0026]FIG. 1 depicts the non-limiting preferred procedure followed inproducing the inventive low-shrink polypropylene fibers. The entirefiber production assembly 10 comprises an extruder 11 comprising fourdifferent zones 12, 14, 16, 18 through which the polymer (notillustrated) passes at different, increasing temperatures. The moltenpolymer is mixed with the nucleator compound (also molten) within amixer zone 20. Basically, the polymer (not illustrated) is introducedwithin the fiber production assembly 10, in particular within theextruder 11. The temperatures, as noted above, of the individualextruder zones 12, 14, 16, 18 and the mixing zone 20 are as follows:first extruder zone 12 at 205° C., second extruder zone 14 at 215° C.,third extruder zone 16 at 225° C., fourth extruder zone 18 at 235° C.,and mixing zone 20 at 245° C. The molten polymer (not illustrated) thenmoves into a spin head area 22 set at a temperature of 250° C. which isthen moved into the spinneret 24 (also set at a temperature of 250° C.)for strand extrusion. The fibrous strands 28 then pass through a heatedshroud 26 having an exposure temperature of 180° C. The speed at whichthe polymer strands (not illustrated) pass through the extruder 11, spinpack 22, and spinneret 24 is relatively slow until the fibrous strands28 are pulled through by the draw rolls 32, 34, 38. The fibrous strands28 extend in length due to a greater pulling speed in excess of theinitial extrusion speed within the extruder 11. The fibrous strands 28are thus collected after such extension by a take-up roll 32 (set at aspeed of 370 meters per minute) into a larger bundle 30 which is drawnby the aforementioned draw rolls 34, 38 into a single yarn 33. The drawrolls are heated to a very low level as follows: first draw roll 34 68°C. and second draw roll 38 88° C., as compared with the remaining areasof high temperature exposure as well as comparative fiber drawingprocesses. The first draw roll 34 rotates at a speed of about 377 metersper minute and is able to hold fifteen wraps of the polypropylene fiber33 through the utilization of a casting angle between the draw roll 34and the idle roll 36. The second draw roll 38 rotates at a higher speedof about 785 meters per minute and holds eight wraps of fiber 33, andthus requires its own idle roll 40. After drawing by these coldtemperature rolls 34, 38, the fiber is then heat-set by a combination oftwo different heat-set rolls 42, 44 configured in a return scheme suchthat eighteen wraps of fiber 33 are permitted to reside on the rolls 42,44 at any one time. The time of such heat-setting is very low due to alow amount of time in contact with either of the actual rolls 42, 44, soa total time of about 0.5 seconds is standard. The temperatures of suchrolls 42, 44 are varied below to determine the best overall temperatureselection for such a purpose. The speed of the combination of rolls 42,44 is about 1290 meters per minute. The fiber 33 then moves to a relaxroll 46 holding up to eight wraps of fiber 33 and thus also having itsown feed roll 48. The speed of the relax roll 46 is lower than theheat-set roll (1280 meters per minute) in order to release some tensionon the heat-set fiber 33. From there, the fiber 33 moves to a winder 50and is placed on a spool (not illustrated).

[0027] Inventive Fiber and Yarn Production

[0028] The following non-limiting examples are indicative of thepreferred embodiment of this invention:

[0029] Yarn Production

[0030] Yarn was made by compounding Amoco 7550 fiber grade polypropyleneresin (melt flow of 18) with a nucleator additive and a standard polymerstabilization package consisting of 500 ppm of Irganox® 1010, 1000 ppmof Irgafos® 168 (both antioxidants available from Ciba), and 800 ppm ofcalcium stearate. The base mixture was compounded at 2500 ppm in a twinscrew extruder (at 220° C. in all zones) and made into pellets. Theadditive was selected from the group of three polypropylene clarifierscommercially available from Milliken & Company, Millad® 3905 (DBS),Millad® 3940 (p-MDBS sorbitol), Millad® 3988 (3,4-DMDBS), twopolypropylene nucleators commercially available from Asahi-DenkaChemical Company (NA-11 and NA-21), sodium benzoate, Liquid 3,4-DMDBS,and 1,3:2,4-bis(2,4,5-trimethylbenzylidene) sorbitol (2,4,5-TMDBS).

[0031] The pellets were then fed into the extruder on an Alex James &Associates fiber extrusion line as noted above in FIG. 1. Yarn was spunwith the extrusion line conditions shown in Table 1 using a 68 holespinneret, giving a yarn of nominally 150 denier. For each additive,four yarns were spun with heat-set temperatures of 100°, 110°, 120°, and130° C. respectively. These temperatures are the set temperatures forthe controller for the rolls 42, 44. In practice, a variation is foundto exist over the surface of the rolls 42, 44, up to as much as 10° C.Pellets with no nucleator additive were used to make control fibers.

[0032] The yarns were tested for shrinkage in boiling water by cutting alength of yarn, marking the ends of a 10″ section with tape, placing theyarn in boiling water for 5 minutes, then taking the yarn out andmeasuring the length of the section between the tape marks. Measurementswere taken on five pieces of each yarn, and the average change indimension is divided by the initial length (10 inches) to give %shrinkage. Also, the measurements below have a statistical error of ±0.4percentage units.

[0033] The yarns were similarly tested for shrinkage in hot air at 150°C. and 130° C. by marking a 10″ section of yarn, placing it in an ovenfor five minutes at the measurement temperature, and similarly measuringthe % shrinkage after removing the yarn from the oven. Again, fivesamples were measured, and the average shrinkage results are reportedfor each sample in Table 1. The shrink measurements are listed below thetested nucleators for each yarn sample. The yarn samples were asfollows: POLYPROPYLENE YARN COMPOSITION TABLE Yarn Samples with SpecificNucleators Added Yarn Sample Nucleator Added A NA-11 B NA-21 C SodiumBenzoate D DBS E p-MDBS F 3,4-DMDBS G Liquid 3,4-DMDBS H 2,4,5-TMDBS I(Comparative) None (Control)

[0034] Fiber and Yarn Physical Analyses

[0035] These sample yarns were then tested for shrink characteristicswith a number of different variables including heat-set temperaturesdifferences (on the heat-set rolls) during manufacture and differentheat-exposure conditions (hot air at various temperatures and boilingwater exposure at temperatures in excess of 100° C.). The results aretabulated below: EXPERIMENTAL TABLE 1 Experimental Shrink Measurementsfor Sample Yarns Shrinkage Test Sample Yarn Heatset Temp. (° C.) andTemp. .(° C.) Shrinkage A 100 150 Hot air 9.5% A 110 150 Hot air 9.4% A120 150 Hot air 8.1% A 130 150 Hot air 6.7% A 100 130 Hot air 7.4% A 110130 Hot air 5.9% A 120 130 Hot air 4.9% A 130 130 Hot air 4.0% A 100Boiling water 4.9% A 110 Boiling water 4.1% A 120 Boiling water 3.6% A130 Boiling water 2.7% B 100 150 Hot air 11.1% B 110 150 Hot air 10.1% B120 150 Hot air 9.3% B 130 150 Hot air 6.7% B 100 130 Hot air 8.1% B 110130 Hot air 7.3% B 120 130 Hot air 6.3% B 130 130 Hot air 3.4% B 100Boiling water 5.6% B 110 Boiling water 4.7% B 120 Boiling water 2.7% B130 Boiling water 2.3% C 100 150 Hot air 10.9% C 110 150 Hot air 11.2% C120 150 Hot air 9.5% C 130 150 Hot air 7.1% C 100 130 Hot air 7.8% C 110130 Hot air 7.4% C 120 130 Hot air 6.2% C 130 130 Hot air 4.5% C 100Boiling water 6.0% C 110 Boiling water 5.0% C 120 Boiling water 3.9% C130 Boiling water 2.6% D 100 150 Hot air 9.8% D 110 150 Hot air 9.7% D120 150 Hot air 9.5% D 130 150 Hot air 5.8% D 100 130 Hot air 7.4% D 110130 Hot air 6.9% D 120 130 Hot air 6.2% D 130 130 Hot air 2.9% D 100Boiling water 5.6% D 110 Boiling water 4.5% D 120 Boiling water 3.1% D130 Boiling water 2.1% E 100 150 Hot air 10.9% E 110 150 Hot air 9.2% E120 150 Hot air 8.0% E 130 150 Hot air 4.0% E 100 130 Hot air 7.5% E 110130 Hot air 6.1% E 120 130 Hot air 4.5% E 130 130 Hot air 2.7% E 100Boiling water 4.6% E 110 Boiling water 4.0% E 120 Boiling water 2.4% E130 Boiling water 1.9% F 100 150 Hot air 13.6% F 110 150 Hot air 12.4% F120 150 Hot air 7.3% F 130 150 Hot air 7.2% F 100 130 Hot air 9.2% F 110130 Hot air 8.0% F 120 130 Hot air 3.7% F 130 130 Hot air 3.4% F 100Boiling water 6.5% F 110 Boiling water 4.0% F 120 Boiling water 2.6% F130 Boiling water 2.7% G 100 150 Hot air 12.9% G 110 150 Hot air 11.7% G120 150 Hot air 9.3% G 130 150 Hot air 7.6% G 100 130 Hot air 9.2% G 110130 Hot air 8.8% G 120 130 Hot air 6.5% G 130 130 Hot air 4.3% G 100Boiling water 6.0% G 110 Boiling water 5.3% G 120 Boiling water 3.9% G130 Boiling water 2.8% H 100 150 Hot air 12.2% H 110 150 Hot air 10.9% H120 150 Hot air 9.6% H 130 150 Hot air 6.8% H 100 130 Hot air 8.9% H 110130 Hot air 8.0% H 120 130 Hot air 6.3% H 130 130 Hot air 3.0% H 100Boiling water 5.5% H 110 Boiling water 4.7% H 120 Boiling water 3.3% H130 Boiling water 2.1% I 100 150 Hot air 21.3% I 110 150 Hot air 19.3% I120 150 Hot air 17.4% I 130 150 Hot air 13.4% I 100 130 Hot air 12.5% I110 130 Hot air 10.7% I 120 130 Hot air 8.6% I 130 130 Hot air 5.3% I100 Boiling water 6.8% I 110 Boiling water 5.2% I 120 Boiling water 3.2%I 130 Boiling water 3.2%

[0036] In addition, two commercial yarns were obtained from FilamentFiber Technology and tested in each of the three tests, with the resultsshown in Table 3. Commercial Yarn #1 is an air jet textured yarn with ablack pigment. Commercial Yarn #2 is an air jet textured yarn with awhite pigment. EXPERIMENTAL TABLE 2 Experimental Data for ComparativeCommercial Polypropylene Yarns Test Comm. Yarn #1 Comm. Yarn #2 150° C.Hot air shrinkage 13.0% 12.1% 130° C. Hot air shrinkage 7.8% 7.0%Boiling water shrinkage 4.8% 5.5%

[0037] It is evident from these two TABLEs that the inventivepolypropylene yarns (including those made from the inventive methoddescribed above) exhibit vastly improved shrinkage rates for all threetest methods and thus are clearly improvements over the commerciallyavailable prior art yarns as well as those yarns lacking nucleatingagent and heat-set.

[0038] Additive Level Dependence

[0039] To test the dependence on nucleator additive level, additionalyarns were spun in accordance with the method described above withvarying levels of additive using Amoco 7550 resin. The additive wascompounded into the resin and the fibers spun under the same conditionsas in the previous examples. The yarns were similarly tested, with theresults shown in Table 5. POLYPROPYLENE YARN SAMPLE TABLE Yarn Sampleswith Specific Nucleators Added Yarn Sample Nucleator Added (Amount ppm)J NA-11 (1000) K 3,4-DMDBS (1250) L 2,4,5-TMDBS (1250)

[0040] EXPERIMENTAL TABLE 3 Experimental Data for Different NucleatorLevels in Polypropylene Yarns Shrinkage Test Sample Yarn Heatset Temp.(° C.) and Temp.. (° C.) Shrinkage J 100 150 Hot air 18.1% J 110 150 Hotair 16.6% J 120 150 Hot air 16.7% J 130 150 Hot air 9.0% J 100 130 Hotair 10.4% J 110 130 Hot air 9.0% J 120 130 Hot air 6.8% J 130 130 Hotair 4.5% J 100 Boiling water 5.4% J 110 Boiling water 4.8% J 120 Boilingwater 3.3% J 130 Boiling water 2.6% K 100 150 Hot air 15.7% K 110 150Hot air 17.1% K 120 150 Hot air 13.0% K 130 150 Hot air 8.8% K 100 130Hot air 9.3% K 110 130 Hot air 8.6% K 120 130 Hot air 5.5% K 130 130 Hotair 4.0% K 100 Boiling water 6.8% K 110 Boiling water 4.5% K 120 Boilingwater 3.3% K 130 Boiling water 2.5% L 100 150 Hot air 16.9% L 110 150Hot air 15.8% L 120 150 Hot air 13.2% L 130 150 Hot air 8.7% L 100 130Hot air 11.1% L 110 130 Hot air 9.2% L 120 130 Hot air 6.8% L 130 130Hot air 4.5% L 100 Boiling water 6.8% L 110 Boiling water 4.3% L 120Boiling water 3.3% L 130 Boiling water 2.3%

[0041] Thus, additive levels are important to providing overall good lowshrinkage characteristics for the target polypropylene yarns. Higherlevels appear to provide better shrinkage properties.

[0042] X-ray Scattering Analysis

[0043] The long period spacing of several of the above yarns was testedby small angle x-ray scattering (SAXS). The small angle x-ray scatteringdata was collected on a Bruker AXS (Madison, Wis.) Hi-Star multi-wiredetector placed at a distance of 105 cm from the sample in an Anton-Paarvacuum chamber where the chamber was evacuated to a pressure of not morethan 100 mTorr. X-rays (λ=1.54178 Å) were generated with a MacSciencerotating anode (40 kV, 40 mA) and focused through three pinholes to asize of 0.2 mm. The entire system (generator, detector, beampath, sampleholder, and software) is commercially available as a single unit fromBruker AXS. The detector was calibrated per manufacturer recommendationusing a sample of silver behenate.

[0044] A typical data collection was conducted as follows. To preparethe sample, the yarn was wrapped around a 3 mm brass tube with a 2 mmhole drilled in it, and then the tube was placed in an Anton-Paar vacuumsample chamber on the x-ray equipment such that the yarn was exposed tothe x-ray beam through the hole. The path length of the x-ray beamthrough the sample was between 2-3 mm. The sample chamber and beam pathwas evacuated to less than 100 mTorr and the sample was exposed to theX-ray beam for one hour. Two-dimensional data frames were collected bythe detector and unwarped automatically by the system software. The datawere smoothed within the system software using a 2-pixel convolutionprior to integration. To obtain the intensity scattering data [I(q)] asa function of scattering angle [2θ] the data were integrated over φ withthe manufacturer's software set to give a 2θ range of 0.2°-2.5° inincrements of 0.01° using the method of bin summation. These rawscattering data were then transformed into a real space correlationfunction K(z) using a FORTRAN program written in house to evaluate theintegral:K(z) = ∫₀^(∞)4π  q²I(q)cos (2π  qz)q  where  q = 4πsin(θ)/λ.

[0045] The integral was evaluated by direct summation over all values 2

in the data range (0.20°-2.5°) and over the real space values from 0nm-50 nm. This follows the method of G. Strobl (Strobl G. The Physics ofPolymers; Springer: Berlin 1997, pp. 408-14), entirely incorporated byreference. From the one-dimensional correlation function, K(z), one canextract the morphological data of interest, in this case long periodspacing (L). The integrated intensity data I(q) as a function of 2θdemonstrates a broad hump corresponding to the long period spacing (FIG.2). The K(z) function has a characteristic shape (FIG. 3). The relevantextractable data points are indicated. Long-period spacing is extractedfrom K(z) data as the global maximum of the function occurring at ahigher z value than the global minimum.

[0046] These data are collected in Table 6. Also included in Table 6 arethe measurements as a result of 150° C. hot air exposure (to test forshrinkage). As can be clearly seen, a longer SAXS long periodcorresponds to a lower shrinkage. In addition, samples prepared with theadditive, but without sufficient heat in the process (represented inthis case by a 130° C. heatset), gave a smaller SAXS long period and acorrespondingly higher 150° C. hot air shrinkage. The following TABLEthus shows the correlation between SAXS long period measurements with150° C. hot air exposure (for shrinkage of the target yarns), as well asthe correlation between heat-set temperatures with such characteristics.EXPERIMENTAL TABLE 4 SAXS and 150° C. Hot Air Shrinkage Data For YarnSamples Sample Yarn Heat-set Temp. (° C.) Shrinkage SAXS Long Period A130 6.7% 26.45 B 130 6.7% 22.35 C 130 7.1% 21 D 130 5.8% 23.2 E 130 4.0%26.4 E 120 8.0% 21 E 110 9.2% 18.4 F 130 7.2% 21.55 H 130 6.8% 22.4Comm. Yarn 1 — 12.1% 16.95 Comm. Yarn 2 — 13.0% 15.6

[0047] It is thus evident that the higher the long period as measured bysmall-angle X-ray scattering, the lower the shrinkage exhibited by thetarget polypropylene yarn.

[0048] Peak Crystallization Temperatures

[0049] As noted above, in order to provide the desired low-shrinkcharacteristics to the target yarns and/or fibers, a nucleating agentshould be added. Although the presence of a nucleating agent or agentsis necessary to accord such low-shrink properties in tandem with aproper heat-setting of the fiber and/or yarn, it is not a requirementthat all nucleating agents present within the target yarn and/or fiberexhibit a relatively high peak crystallization temperature. There arecertain instances, however, wherein the nucleating agent does inducesuch high peak crystallization temperatures and thus their presence maybe determined through differential scanning calorimetry analysis. Forthose nucleating agents that do not induce the target polymer to exhibitsuch high peak crystallization temperatures, other methods of analysis(gas chromatography/mass spectroscopy, as one example) may be utilizedto determine their presence. For example, although sodium benzoate iswell known as a polyolefin nucleating agent (as defined above), peakcrystallization results within polypropylene yarns and/or fibers are notconsistent with accepted results for sodium benzoate within other typesof polypropylene articles (such as plaques, containers, and the like).Some peak crystallization measurements for sodium benzoate withinpolypropylene fibers have been nearly as low as the measurements for thepolypropylene itself. Again, since sodium benzoate provides effectivelow-shrink characteristics for such fibers and/or yarns, the lack ofhigh peak crystallization temperatures for such sodiumbenzoate-containing polypropylene fiber samples does not remove sodiumbenzoate from the definition of nucleating agent for the purposes ofthis invention.

[0050] Thus, for the polypropylene samples including the remaining typesof nucleating agents, peak crystallization was measured by the followingmethod (a modified version of ASTM D3417-99 including a manner ofcreating a proper measurable sample of the test fibers themselves): APerkin-Elmer DSC7 calibrated with an indium metal standard at a heatingrate of 20° C./min was used to measure the peak crystallizationtemperature of the polypropylene fibers. Bundles of polypropylene fiberswere heated to 220° C. for 1 minute and then compressed into thin disksapproximately 250 μm thick. The specific polyolefin/DBS mixturecomposition was heated from 60° C. to 220° C. at a rate of 20° C. perminute to produce a molten formulation and held at the peak temperaturefor 2 minutes. At that time, the temperature was then lowered at a rateof 20° C. per minute until it reached the starting temperature of 60° C.The peak crystallization temperature of the polymer was thus measured asthe peak maximum during the crystallization exotherm. This entireprocedure of first preparing fibers into plaques followed by DSCanalysis in accordance with the modified ASTM D-3417-99 test is hereinreferred to as “fiber peak crystallization temperature measurement(s)”for the purposes of this invention. The results for the fiber peakcrystallization temperature measurements for the samples from Table 1,above, are tabulated below (with a standard deviation of ±0.5° C.):EXPERIMENTAL TABLE 5 Peak Crystallization Temperatures For Yarn SamplesPeak Crystallization Sample Yarn Heat-set Temp. (° C.) Temperature (Tc)(° C.) A 120 124.3 B 130 124.6 D 130 117.0 E 130 123.7 F 130 124.5 H 130122.2 I (Comparative) 130 109.9

[0051] Thus, the presence of certain nucleating agents providedrelatively high peak crystallization temperatures for the sample yarns(at least above 115° C., and as high as a low level of about 117.0° C.).

[0052] Fabric Article Production and Analyses

[0053] Woven Fabric Comprising the Inventive Yarn

[0054] Fabric was woven using the inventive yarns and a 150 denier, 34filament polyester warp, and weaving a square weave with 84 picks/inchusing five yarns: a control made as above with no additive with finaldraw roll 3A and 3B temperatures of 110° C. and 130° C. Threeexperimental yarns were made having 2500 ppm 3,4-DMDBS (Sample yarns F,from above) and a final draw roll 3A and 3B temperature of 110° C., 130°C., and 140° C respectively. These sample fabrics were separated into 18inch squares. A 12″ box was drawn in the center of the piece of fabric,and the fabric was washed five times in either hot (60° C.) or cold (20°C.) water, and dried for 30 minutes in a conventional dryer (at about70° C. for 20 minutes). The dimensional change of the 12″ box wasmeasured, and is reported in Table 6 as % shrinkage. EXPERIMENTAL TABLE6 Fabric Sample Shrinkage Data Sample Fabric (corresponding YarnHeat-set Cold Wash Hot Wash to TABLE 1, above) Roll Temp. (° C.)Shrinkage Shrinkage F 110 2.4% 5.8% F 130 2.9% 3.7% F 140 2.4% 3.7% I(Comparative) 110 8.9% 14.9% I (Comparative) 130 5.0% 6.8%

[0055] Thus, it is evident that the fabric samples comprising theinventive yarns exhibit lower shrinkage rates as well.

[0056] Knit Fabric Construction Comprising the Inventive Yarn

[0057] Yarns from TABLE 1 were produced with a heat-set roll temperatureof 130° C. and were subsequently knit into socks on a Lawson HemphillFAK Knitter 36 gage knitting machine using 160 needles (needle no.71.70) at speed setting 4 using 40 PSI of air pressure. The fabric waslaid flat, and a 2.75″10″ section of sock was marked (10″ in the coursedirection, 2.75″ in the wales direction). The socks were placed in anoven at 150° C. (hot air) for five minutes, and then the dimensions ofthe marked section were measured. The shrinkage in each direction andthe area shrinkage are reported in TABLE 8, below. The area shrinkage isthe product of the measured dimensions (the area) divided by 27.5 sq.inches (the original area), reported as a percentage. EXPERIMENTAL TABLE7 150° C. Hot Air Shrinkage Data For Knit Fabric Samples Sample YarnCourse Shrinkage Wales Shrinkage Area Shrinkage A 5.3% 2.8% 8.0% B 7.2%2.8% 9.8% C 8.8% 2.2% 10.8% D 0.6% 3.4% 4.0% E 1.6% 1.6% 3.2% F 5.6%2.8% 8.2% H 7.2% 2.2% 9.2% I (Comparative) 11.3% 4.4% 15.2% Comm. Yarn 120.6% 5.3% 24.8% Comm. Yarn 2 20.0% 3.8% 23.0%

[0058] Therefore, it is evident that the inventive knit fabrics exhibitfar better shrinkage characteristics than the commercial yarn-containingfabric samples as well as the control without any nucleator compoundpresent. The control yarn gave very high area shrinkage, which waseclipsed by the air jet textured commercial yarns. Yarns with DBS andp-MDBS gave very low shrinkage, easily acceptable within the apparelindustry.

[0059] Non-Woven Fabric Construction Comprising the Inventive Yarn

[0060] Yarns from Sample E of TABLE 1 were produced with a heat-set rolltemperature of 130° C. and were extruded at a pump rate of 87.6 cc/minwith a 68 hole spinneret, to give a total yarn denier of 680 and adenier per filament of 10. The fibers were combined by plying such into5 yarns of 2720 denier, which were then combined into a single tow of13600 denier, which was heated at ˜90° C. in steam, crimped in a stufferbox, and then cut to a staple length of 3.25 inches. The staple was thencarded, lapped using a Fiber Locker manufactured by James Hunter MachineCompany, and then needled with a Di-Lour-6 manufactured by Dilo, Inc.into a bat approximately 12×24 inches. Boxes of 130.3 cm² were marked onthe bat. The bat was then molded by heating with an IR lamp for 60seconds to temperatures reaching 120-150° C. and then compressing in a10° C. mold. The boxes showed average shrinkage of 3.2%.

[0061] A control yarn of 10 DPF with no additive was obtained. It wasthen crimped and cut into staple, carded, lapped, and needled in thesame manner. Boxes were again marked prior to molding. When molded underthe same conditions, the boxes showed an average shrinkage of 11.7%.

[0062] It is thus evident that the non-woven fabrics made from theinventive low-shrink propylene yarns also exhibit excellent low-shrinkcharacteristics in comparison with control samples.

[0063] There are, of course, many alternative embodiments andmodifications of the present invention which are intended to be includedwithin the spirit and scope of the following claims.

What we claim is:
 1. A method of producing a polypropylene fiber, saidmethod comprising the sequential steps of a) providing a polypropylenecomposition in pellet or liquid form comprising at least 10 ppm of anucleating agent; b) melting and mixing said polypropylene compositionof step “a” to form a substantially homogeneous molten plasticformulation; c) extruding said plastic formulation to form a fiberstructure, d) mechanically drawing said fibers; and e) exposing saiddrawn fiber of step “d” to a temperature of at least 110° C.; whereinthe temperature at which step “d” is performed is below that at whichstep “e” is performed.
 2. A method of producing a polypropylene fiber asin claim 1, wherein the temperature in the drawing step “d” is less than100° C.
 3. A method of producing polypropylene fiber as in claim 2,wherein the fiber is exposed to a high temperature in step “e” for atmost 1 second.
 4. A method of producing polypropylene fiber as in claim1, wherein the fiber exhibits a shrinkage at 150° C. in hot air of atmost 7%.
 5. The method of producing polypropylene fiber as in claim 1,wherein the fiber exhibits a shrinkage at 100° C. in boiling water of atmost 3%.
 6. The method of producing polypropylene fiber as in claim 1,wherein said nucleating agent of step “a” is present in an amount of atleast 100 ppm.
 7. The method of producing polypropylene fiber as inclaim 6, wherein said nucleating agent of step “a” is present in anamount of at least 1250 ppm.
 8. The method of claim 1 wherein saidnucleating agent of step “a” is selected from the group consisting ofp-MDBS, 3,4-DMDBS, 2,4,5-TMDBS, Liquid 3,4-DMDBS, DBS, sodium benzoate,NA-11, NA-21, and any mixtures thereof.
 9. The method of claim 8 whereinsaid nucleating agent is p-MDBS.